1. Field of the Invention
This invention relates to mixers in general and more particularly to a double balanced mixer that provides increased isolation.
2. Description of the Prior Art
A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates intermodulation products due to the non-linearity response.
Isolation is a measure of the circuit balance within the mixer. When the isolation is high, the amount of leakage or feed through between the mixer ports will be very small. Typically, isolation decreases as frequency increases due to the unbalance in the mixer circuit. Isolation can be measured as LO-RF isolation which is the amount the LO drive level is attenuated when it is measured at the RF port. LO to IF isolation is the amount the LO drive level is attenuated when it is measured at the IF port.
Mixers are typically designed with one of three topologies: single ended, balanced, and double balanced. The double balanced mixers are capable of isolating both the RF signal and the local oscillator LO voltages from the output and thus allow overlap of the RF and IF frequency bandwidths. Several prior art mixer circuits are well known. One mixer design uses a schottky diode quad or ring circuit that uses four diodes with all of the diodes pointed in the same direction. Another mixer circuit is called a star circuit, which uses two diodes pointing toward the central node and two diodes pointing away from the central node.
Double balanced and double-double balanced mixers have been described in the prior art. Diode-type double-balanced mixers belong to the general classification of “Resistive Switching” mixers. Referring to FIG. 1, a prior art four diode mixer 100 is shown. In this type of mixer, a local oscillator input signal is applied to input port LO that is sufficiently large to cause strong conduction of the alternate diode pairs D101 and D102 or D103 and D104, thereby changing them from a low to a high resistance state during each half of the LO cycle. A virtual ground is, therefore, switched or commutated between the radio frequency/intermediate frequency (RF/IF) transformer T102 windings at a rate corresponding to the LO frequency. Since this switching causes a 180 degree phase reversal of the RF to IF port transmission during each half of the LO cycle, the mixing process is called bi-phase modulation. For low frequency operation, these devices typically use ferrite core flux coupled transformers which exhibit leakage inductance and stray capacitance which limits their upper frequency operation and have poor isolation. Using the mixer of FIG. 1, overlapping RF-IF or LO-IF frequency coverage is very difficult to attain because the IF output encounters both the RF and LO structures in series for the IF signal path.
Turning now to FIG. 2, another prior art mixer is shown. A more complex eight diode mixer 200 is shown that produces an overlapping IF range. Mixer 200 has two diode rings DR202 and DR204 that are coupled together. Examination of this mixer structure reveals that the LO is switching two diode pairs at a time which are in series with the RF-IF signal path. By tracing out the RF to IF signal connections for each half of the LO input cycle, we see that bi-phase modulation is again being performed. The IF port can be seen to be an RF and LO null. The principle advantage of this design is its large RF-IF range overlap, but with twice as many diodes it requires more LO drive.
These basic mixer types can be further sub-divided into categories by the nature of their mixing elements as follows:                Class I. The most common design consists of a pair (or more) of the ferrite-core wideband transformers with four diodes connected in a “ring” configuration. Nominally, these components require about +7 dBm LO drive power.        Class II, Type 1. This type also uses the ring topology with two series-connected diodes in each are. The eight diodes may be similar or different. LO drive levels typically range from +13 to +17 dBm.        Class II, Type 2. These rely on a ring connection, but feature a precision resistor in series with a single diode in each arm. These four-diode designs are typically driven at +17 dBm.        
Class III. These are essentially Class II, Type 2 circuits with a large capacitor connected in parallel with the precision series resistor, and they are driven by LO signal in the +20 to +30 dBm range.                Class IV. Termination Insensitive Mixers. This mixer circuit, called TIM, consists of a transmission line hybrid network driving two sets of diodes. Isolation between each hybrid's opposite ports allows the LO to independently control the switching action of alternately conducting diode sets.        
Referring now to FIG. 3, another prior art double-double balanced mixer 300 is shown. Mixer 300 has a LO balun B1 connected to a pair of ring diodes DR1 and DR2. The LO balun has an input port LO and a pair transformers T1 and T2. An RF input port RF is coupled to an RF balun B2. RF balun B2 has a pair of transformers T3 and T4. Ring diode DR1 has diodes D1, D2, D3 and D4. Ring diode DR2 has diodes D5, D6, D7 and D8. The IF output port IF is coupled to the junction of diodes D1, D4 and D5, D8. The mixer of FIG. 3, is better suited to low frequency operation and does not have high LO to RF or LO to IF isolation over a wide frequency range.
While double balanced mixers have been used, they have suffered from not being able to provide high isolation over a wide frequency range. A current unmet need exists for an improved double balanced mixer that has high isolation over a wide frequency range.